Advances in polymerization and catalysis have resulted in the ability to produce many new polymers having improved physical and chemical properties useful in a wide variety of superior products and applications. With the development of new catalysts, the choice of polymerization (solution, slurry, high pressure or gas phase) for producing a particular polymer have been greatly expanded. Also, advances in polymerization technology have provided more efficient, highly productive and economically enhanced processes. Especially illustrative of these advances is the development of the technology field utilizing metallocene catalyst systems.
As with any new technology field, particularly in the polyolefins industry, a small savings in cost often determines whether a commercial endeavor is even feasible. This aspect of the metallocene technology field is evident by the number of participants in the industry looking for new ways to reduce cost. In particular, there has been tremendous focus in the industry on developing new and improved metallocene catalyst systems. Some have focused on designing the catalyst systems to produce new polymers, others on improved operability, and many more on improving catalyst productivity. The productivity of a catalyst, that is, the amount of polymer produced per gram of the catalyst, usually is the key economic factor that can make or break a new commercial development in the polyolefin industry. Reactor operability—lack of fouling and sheeting, etc., of the polymerization reactor—is also a major concern for polyolefin producers. Reducing the occurrence of reactor fouling has commercial benefits in reduced down time for the reactor and improved output of polyolefin resin, as well as higher quality resin.
From the early stages in the metallocene technology field, beginning with the discovery of the utility of alumoxane as a cocatalyst in the early 1980's, to the discovery of substitutions on the bulky ligands of the metallocene compounds, through the development of non-coordinating anions, and today with the ever-increasing number of new metallocene bulky ligand compounds, catalyst productivity has been a primary focus.
Considering the discussion above, there is still a need for higher productivity catalyst systems capable of providing the efficiencies necessary for implementing commercial polyolefin processes. Further, it has been found, especially in gas phase fluidized bed processes, that reactor performance (presence or absence of reactor fouling, sheeting, etc.) is an issue when using supported metallocene catalysts. Secondary additives or support “surface modifiers” are often used to reduce fouling and hence improve commercial performance of the reactor. Addition of these surface modifiers, however, adds cost and complexity to the polymerization process. Thus, it would be highly advantageous to have a polymerization process and catalyst system capable of producing polyolefins with improved catalyst productivities and reactor performance.
While the present invention is susceptible to various modifications and alternative forms, specific exemplary embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.